Baby humans get the shits when they're teething so it's perfectly feasible that a puppy could to. Puppies also eat a lot of unsavoury stuff when out in the world so shit happens. If it becomes a more common occurrence then plain boiled rice for a couple of days will probably resolve the issue. Include some boiled chicken with the rice then revert to it's normal food. This time of year is bad for my old dog's guts. We usually have to resort to rice/chicken and rice around Christmas. It seems to coincide with increased fox (sexy time) action down the end of the garden. I think there may be some sly poo eating going on when he's out of sight.